1. Field of the Invention
This invention relates to a safety device for an oil hydraulic braking system particularly of an automobile.
2. Description of the Prior Art
Generally, in the braking system of an automobile, in view of the running stability at the time of braking, it is provided that the oil hydraulic pressure supplied to the rear wheel cylinder is a little lower than that supplied to the front wheel cylinder when the brake is operated by an oil hydraulic pressure higher than a predetermined level. Thus, the braking force applied to the rear wheels is somewhat weaker than that applied to the front wheels. Such a function of lowering the braking force applied to the rear wheels as compared with that applied to the front wheels at a predetermined ratio is generally called "proportioning." This is effected by supplying the oil hydraulic pressure delivered from a master cylinder of the brake, especially a rear master cylinder in the case of a dual-type oil hydraulic braking system, including independent oil hydraulic circuits for the front and rear wheels, to the rear wheel cylinder through a pressure reducing element called a proportioning valve. Thus, the oil hydraulic pressure in the master cylinder is transmitted to the rear wheel cylinder in a reduced increasing rate when the oil hydraulic pressure in the master cylinder is above a predetermined level. However, in the case of the dual circuit oil hydraulic braking system, it is desirable, that if the front brake fails to operate due to a mishap caused in the oil hydraulic circuit for the front wheel, the rear brake operates with a much larger operating force to compensate for the failure of the front brake. Therefore, it is adverse to such a requirement that the oil hydraulic pressure supplied from the rear master cylinder to the rear wheel cylinder is specifically reduced by the proportioning valve in the situation where the front brake oil hydraulic circuit has failed. Therefore, it is desirable that the proportioning valve is adapted to be selectively by-passed so that the oil hydraulic pressure delivered from the rear master cylinder is directly supplied to the rear wheel cylinder when the front brake fails to operate due to a mishap caused in the oil hydraulic circuit for the front brake.
With regard to the dual-type oil hydraulic circuit including individual oil hydraulic circuits for the front and rear wheels, it is very desirable, with a view toward preventing accidents that a failure in either of the oil hydraulic circuits for the front or rear wheel is detected and the driver is pre-warned by a proper warning means before said failure is actually noticed by the driver because of a distinct deterioration of the performance of the automobile at the time of braking. Of course, it is also desirable that a malfunction of the proportioning valve is detected by a proper detecting means and the driver appropriately warned before the malfunction manifests itself as a change in the automobile running performance at the time of braking.
To meet with the aforementioned requirements, a control valve as disclosed in U.S. Pat. No. 3,448,230 has been proposed. In said U.S. patent, a piston valve including a floating piston valve member is provided, wherein the oil hydraulic pressure delivered from the front master cylinder and that delivered from the rear master cylinder are supplied to first and second ports provided at opposite end regions of the piston valve member to be applied at opposite ends thereof so that the two oil hydraulic pressures balance with each other when the front and rear master cylinders are operating normally, thereby holding the piston valve member at its neutral position. Adjacent said second port where the oil hydraulic pressure from the rear master cylinder is supplied, there is provided a third port which communicates with a rear wheel cylinder and is normally isolated from said second port by an end land portion of the piston member, so that in a normal operating condition, the third port is supplied with an oil hydraulic pressure which is a modification of the oil hydraulic pressure from the rear master cylinder, said modification being effected by a proportioning valve. In this valve structure, if the oil hydraulic pressure from the front master cylinder has failed, the piston valve member is displaced toward said first port due to an unbalance of the pressures applied at opposite ends thereof, whereby said third port communicates directly to said second port due to the displacement of said end land portion, and thus, the oil hydraulic pressure delivered from the rear master cylinder is directly supplied to the rear wheel cylinder while bypassing the proportioning valve. However, in this valve structure, since the piston valve member is adapted to be maintained in its neutral position depending upon a direct balance between the oil hydraulic pressures delivered from the front and rear master cylinders, the positioning of the piston valve member is relatively unstable and tends to readily fluctuate due to a slight unbalance between the two pressures or due to a time delay in the pressure generation. A fluctuation or unstable positioning of the piston valve member might cause a malfunction of the oil hydraulic braking system. If it is desired to overcome this instability by the use of spring means acting at opposite ends of the piston member, as is intended in the valve disclosed in the U.S. Pat. No. 3,448,230, the sensitivity or the accuracy of the operation will be substantially sacrificed. Furthermore, in the case of the valve structure as shown in said U.S. patent, the oil hydraulic pressures delivered from the front and rear master cylinders must be strictly of the same level and performance or, in other words, the front and rear master cylinders must be of substantially the same structure and performance because if such a condition is not satisfied, a stable neutral positioning of the piston valve member cannot be guaranteed.
The control valve disclosed in the aforementioned U.S. patent incorporates a warning means which dispatches a warning signal when the piston valve member has been displaced from its neutral position in either direction due to an unbalance between the oil hydraulic pressures at opposite ends thereof or the pressures delivered from the front and rear master cylinders. A warning signal is thereby produced if a pressure fall occurs in either one of the oil hydraulic circuits of the front and rear brake systems or, more precisely, either one of the oil pressure lines connecting the front and rear master cylinders to said first and second ports, respectively. However, in the valve structure disclosed in the aforementioned U.S. patent, a warning signal is not available if an abnormal pressure drop occurs in the oil hydraulic line leading from the proportioning valve to the rear wheel cylinder, since such a pressure drop does not produce any force to displace the piston valve member in either direction. Furthermore, in said prior structure, a warning signal is not available even when the proportioning valve has become defective, because as previously mentioned, any change in the oil hydraulic pressure applied to said third port does not act to displace the piston valve member in either direction.